Sound processing devices, including hearing aids, ALDs, cochlear implants, and consumer audio devices are being used more frequently in noisy environments. Normally, people make good use of both ears to separate the sounds they want to listen to from the other noises in the environment that they want to ignore. Present day consumer audio devices, hearing aids, cochlear implants, and ALDs also rely on these internal binaural perceptual processes to be able to function adequately in noisy environments. In addition to the internal perceptual processing, many audio devices include various external noise reduction schemes aimed at improving speech intelligibility, sound quality, and listening comfort in noisy environments. These noise reduction schemes typically use information that is available from a single microphone, or an array of closely-spaced microphones that may be worn on one side of the head. They rely on directional information, and spectral and temporal information to separate desired sounds from other noises in the environment. For example, some schemes seek to improve signal-to-noise ratios by expanding the intensity differences between more intense parts of the sound and less intense parts of the sound. A noise reduction scheme based on spectral information may apply more gain to the peaks in the spectrum than to the troughs. A noise reduction scheme based on temporal information may apply more gain at times when the sound is above a certain intensity threshold than when the sound is below this threshold. A noise reduction scheme based on directional information may apply more gain to sounds from the front of the listener than sounds from other directions. There is clear evidence that directional microphones can improve sound quality, comfort, and intelligibility. It is also clear that spectral and temporal noise reduction improves comfort, but the effects of spectral and temporal noise reduction on intelligibility and sound quality are more controversial.
One potential reason for the uncertainty about the effects of external spectral and temporal noise reduction schemes on intelligibility and sound quality is that they are changing the spectral and temporal cues that are used by the internal perceptual processes. If these cues are changed differently in the left and right ears, they may also disrupt the internal binaural processes that most listeners rely upon most heavily in noisy situations. There are at least three important perceptual processes that are important in binaural sound perception:                a) Integration of information from both ears. This includes integration of information about both the desired sounds and the other noises in the environment.        b) The ability to separate sounds from different sources and to pay attention to the sounds from one ear or the other when it is advantageous to do so.        c) The ability to use small timing and intensity differences between the ears.        
Bregman (1990) uses the term “auditory streaming” to describe the perceptual process that separates sounds from different sources and groups together sounds from the same source. A stream is a series of sequential and overlapping sound events that come from the same source. An example of a stream is the speech from a single person speaking. A word or a sentence spoken by this person must be perceived as a connected series of sound events to be understood, while being kept separate from the other sounds in the environment. Important sound events include the onsets and offsets of sounds, and changes in intensity and spectrum. The spectral and temporal noise reduction schemes referred to above introduce onsets and offsets, changes in intensity, and spectral changes in the noise that are correlated with the onsets and offsets and spectral changes in the desired signals. The perceptual effects of introducing these artificial streaming cues are difficult to predict. On one hand, they may emphasize the temporal and spectral characteristics of the desired sounds. On the other hand they will make it more difficult for the internal auditory streaming processes to separate the desired sound events and streams from the noise events and streams.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.